This invention relates generally to touch detection circuits and, more particularly, to detection circuits for use with a matrix arrangement of touch pads in a keyboard.
It has been known to arrange touch pads in a matrix and to utilize multiplexing techniques in order to reduce the number of interconnections extending between the matrix of touch pads and a detection circuit. For a keypad of sixteen keys, it is necessary to provide four drive lines to apply signals to the matrix and four sense lines to sense the touched and non-touched condition of the sixteen touch pads. One problem with such arrangement is that variations in lead length and touch pad layouts cause variations in the sensed signals received on the sense lines depending upon the individual touch pads being touched.
One solution to this problem is proposed in U.S. Pat. No. 4,145,748 issued to Eichelberger et al. for a SELF-OPTIMIZING TOUCH PAD SENSOR CIRCUIT which converts voltages associated with each of the touch pads into digital values, with the digital reading obtained for the no-touch condition for each touch pad being stored in a memory. A control logic circuit cycles through all touch pads and compares the digital readings obtained from each of the touch pads to the value of the no-touch reading for the respective touch pad which has been stored in the memory. When a significant departure from the no-touch condition is obtained, a touch indication is given for that particular touch pad. The control updates the no-touch digital output in the system memory during an optimization mode. The difficulty with the system disclosed in Eichelberger et al. is that the digital output is produced by a charge transfer analog-to-digital converter which is slow, is relatively expensive and requires specialized control signals from the control logic circuit. Furthermore, all compensation techniques are executed in logic because all signals are sensed through a common analog-to-digital converter.
A hardware-oriented solution to the variation in response from one sense line to another is set forth in U.S. Pat. No. 4,413,252 issued to Tyler et al. for a CAPACITIVE SWITCH AND PANEL. In Tyler et al. capacitive trim tabs are provided for each switch to compensate for differences in stray capacitance from switch to switch. Each of the individual tabs requires trimming to provide the appropriate compensation. This approach is both tedious and time consuming.